Comparative analysis of dynamic stability characteristics of T0 and T90 codends

Changing mesh orientation is a key appraoch to enhancing codend selectivity, but it may also alter the hydrodyna-mic performance of the codend. In order to quantify the differences in hydrodynamic and stability between T0 (A diamond mesh) and T90 (A 90° turned mesh) codends, we investigated the hydrodynamic characteristics, geometric shape, and fluttering motions of T0 and T90 codends with different twine diameters under varying flow velocities and catch sizes in a flume tank. Results show that the mesh opening angle in the middle part of T90 codend was approximately 1.2 times larger than of T0 codend. Moreover, the middle section of T90 codend had less shrinkage and greater width than T0 codend. The drag of T90 codend was, on average, larger than that of T0 codend and increased by 1.09 times with the simulative catch size. During the catch stage, the amplitude of the drag oscillations of the codend increased with increasing flow velocity and catch size, with T90 codend exhibiting larger amplitudes than T0 codend. There was no significant correlation between longitudinal displacement amplitude of the empty codend and current speed. Additionally, longitudinal displacement oscillations amplitudes in T0 codend were higher than those in T90 codend. The longitudinal displacement oscillations amplitudes in both codends increased with catch size and current speed, but decreased with twine diameter. The use of a thick diameter T90 codend can improve selectivity and stability, and increase the hydraulic resistance of the codend. The findings of this study can be applied to enhance the hydrodynamic characteristics, selectivity and stability of mid-water trawl codend.